Immunological aging or Immunosenescence and the mechanisms that accelerate it, such as radiation, reduce the host's capability to mount a robust immune response, thus increasing the host's susceptibility to infectious agents and a variety of malignant and non-malignant diseases The Studies of Immunosenescence and Other Late Effects of Acute Radiation Exposure in Atomic Bomb Survivors program supports research into how the late effects of acute radiation exposure and aging affect the process of immunosenescence. The results of the proposed studies on this unique aging population will not only provide a wealth of fundamental biological information on the process of and the impact of radiation on the aging of the immune system, but may also provide insights that lead to amelioration of immune function and other health outcomes in radiation-exposed older adults.